My brother works for US, and I am green with envy for his non-rev status. He has mentioned ID-90's to me that allow non-revs from one airline to travel at steep discounts on other airlines. As he is a fairly new employee with US, he hasn't had the opportunity to use an ID-90 and doesn't know much about them. Can anyone out there give me a little more info about them? How they work? Which airlines participate? Sharing with family? Etc.? Thanks so much.

ID Stands for "Industry Discount" and is indicitive of how much of the fare is discounted, 90/75/25, ect.
Generally airlines set different limits on how they can be used.It depends on the carrier.
If you are jealous of your brothers travel bennies, get an airline job.

The ID-90 fare is calculated based on paying 10% of the full coach (Y) fare in the market you are flying. Each airline has agreements with other airlines. When you fly on another airline rather than your own, it is called "off line pass".

You are boarded after all revenue passengers, after "on-line" stand-by employees (those who work for the airline you are flying), and after the "on-line" stand-by passenger's family members. IE if there is a set left before they shut the door.

ID90s are very useful for domestic US travel to smaller markets or if you don't have a Saturday night stay in your plans. Otherwise, I find that the cheap excursion fares offer confirmed seats at around the same price. I swear by ID90s internationally.

I use ID-90s semi-frequently for travelling from Purdue to Chicago on united Express (Great Lakes Aviation) because American/Eagle doesn't serve LAF. It's quicker and easier to fly from Purdue than to get to Indianapolis, and it costs about the same price. Other than that, the only ID-90 trip I have ever taken was LAX-HNL-SFO on United, and that was because AA flights were all full.